This disclosure concerns a novel composition and method for the solubilization of aromatic lipophiles in water.
Aromatic lipophiles are important ingredients in flavoring many aqueous-based products, such as mouthwashes; at levels as low as 0.1% by weight, they are able to impart a fresh pleasing flavor. However, these aromatic oils are difficult to solubilize in aqueous compositions at levels of 0.2% or greater. Given their hydrophobicity, they tend to remain in a phase separate from the aqueous phase, and so detract from the clear, transparent appearance of a monophasic aqueous composition.
Several conventional solutions to this solubilization problem have been proposed. The first of these is the inclusion of co-solvents, most commonly ethyl alcohol. Conventional mouthwashes commonly have to 10 to 30% by weight of ethanol. Although this is a high level of co-solvent for solubilizing an agent present at only 0.1-1%, ethyl alcohol serves several other functions besides solubilizing flavoring agents. For example, it acts as a cleanser, a surface tension lowering agent, a wetting and penetrating agent, and assists antimicrobial activity of any antiseptic ingredients present in the mouthwash.
Despite these multiple beneficial effects, recent epidemiological studies have reported that ethanol in mouthwashes, especially at higher concentrations, has been implicated in causing oral cancer and in poisoning very young children. Therefore, it is now desirable to omit ethanol or minimize its amounts in mouthwash.
Co-solvent alternatives to ethanol are few in number, especially if the product is one to be ingested. Suitable co-solvents for products to be ingested are limited to glycerine, propylene glycol and other glycols, suitably at levels of 5-30% by weight. As with ethanol, this is a rather high level to effect solubilization of oils present only at 0.2 to 1% by weight. A further drawback is that at levels of 5-30%, glycerine, propylene glycol and other glycols often impart an unpleasant taste to a composition.
Other co-solvents which have conventionally been employed are hydrophilic nonionic surfactants. One such surfactant is a polyethoxylated sorbitan ester (Tweens made by ICI Americas, Inc.). Solubilization of one part of peppermint oil in an aqueous composition requires five (5) parts Tween 80 (polyoxyethylene sorbitan monooleate). In other words, solubilization of 0.5% of an aromatic oil in water requires at least 2.5% by weight Tweens. At these levels, nonionic surfactants impart an undesirably bitter and soapy flavor to the aqueous composition. A further undesirable effect accompanying this level of nonionic surfactant is the reduction of antimicrobial activity of phenolic preservative, commonly present in mouthwashes.
In order to overcome the problem of the bitter, soapy flavor, a nonionic surfactant which has no taste has been proposed for solubilizing aromatic oils. This suffactant, polyoxyethylene-polyoxypropylene block copolymer (Pluronic 127, made by BASF Corporation)imparts no undesirable taste to the product. However, a substantially higher level of this surfactant is required to solubilize the oil than of Tweens: 1 part of aromatic oil in water requires at least 7 parts of this nonionic to effect solubilization. Although tasteless, these nonionic surfactants are thus not efficient solubilizers. Moreover, they share the undesirable side effect with Tweens of inactiveting phenolic germicides such as thymol.
All of the nonionic surfactants conventionally used have a further shortcoming: unlike ethanol, which significantly lowers surface tension, the nonionic surfactants have very little effect on surface tension. In summary, the nonionic surfactant co-solvents do not solubilize as well as ethanol and suffer significant undesirable secondary properties.
Anionic surfactants have also been explored as co-solvents for solubilizing aromatic oils in aqueous compositions. If used at concentrations greater than 0.3% by weight in mouthwashes, anionic surfactants are able to impart a very silky mouth feel to the product. Thus for example, sodium lauryl sulfate has been used alone or in combination with nonionic surfactants as described above. However, anionic surfactants impart a very bitter taste to the aqueous composition and irritate mucous membranes. Moreover, while sodium lauryl sulfate is able to solubilize aromatic oils at levels somewhat lower than nonionic surfactants (3 parts of sodium layryl sulfate to solubilize 1 part of a aromatic oil), an aqueous composition containing 0.2% of oil still requires at least 0.6% of sodium lauryl sulfate for solubilization. These strongly undesirable effects render anionic surfactants unsatisfactory as a co-solvent.
In U.S. Pat. No. 4,150,151, problems associated with mouthwash clarity were addressed by including 5-20% ethanol and 0.1-0.6% of a particular alkyl sulfate anionic surfactant mixture along with 0.1-2% of an essential oil as flavorant. The surfactant mixture consisted essentially of dodecyl (or lauryl) and tetradecyl surfactant salts in a weight ratio of 4:1 to 1:1. The cationic moiety could be chosen from, among others, Na.sup.1+, K.sup.1+, Mg.sup.2+ or NH.sub.4.sup.+, or mixtures thereof, in a water carrier. The composition optionally further included a nonionic emulsifier, and, if desired, 0-2% of an alkali metal halide. The 4:1 to 1:1 ratio of lauryl to tetradecyl was said to be critical to keep the composition "water clear," i.e., free of clouding precipitate after storage at or below 35.degree. F. for extended periods of time. Alternatively, for compositions not to be subjected to cold temperatures, the weight ratio of lauryl to tetradecyl surfactant salts could be 75:1 to 1:1.
U.S. Pat. No. 4,150,151 does not teach or suggest a desirable ratio of magnesium and halide ions (in equivalent amounts to each other) to be provided in a particular ratio to alkali metal ions with alkyl sulfate ions (again in equivalent amounts to each other). The term "equivalent" as used herein has the standard chemical meaning.